Sunday, 8 March 2015

Reflecting on the astronomy behind ‘Moon draws Sun/Earth draws Moon’

Two eclipses are predicted to take place
over the next month. There’s going to be a lunar eclipse on 4th
April which will be visible from that part of the Earth facing away from the
Sun and towards the Moon.

Lunar eclipses are caused by the Moon,
Earth and Sun all being respectively aligned precisely enough so that the Earth
casts its shadow on the face of the Moon. So, out of all the phenomena that we
can observe in the sky, the lunar eclipse is the only one in which the Earth
itself plays a direct role and moves us from being spectators to actors.

Lunar eclipses are visible wherever it is
night time on the Earth. In contrast a total solar eclipse is only visible on
relatively small parts of the Earth. The one that will occurr in a few days time on 20th
March will be total from the Faroe Islands and the far North Atlantic, and Scotland
should see upwards of 94% of the Sun’s light blocked by the Moon.

Eclipses are mesmerising to watch – but they’re
also somewhat disturbing, particular solar ones. Suddenly, the sky darkens, birds
stop singing, animals prepare for night-time, and we can’t help holding our
breath wondering if the Sun will return. And they remind us of the enormous cosmic
coincidence - we see solar eclipses because the Sun and Moon appear to be roughly
the same size when we look at them from the Earth, even though the actual size
of the Sun is 400 times that of the Moon.

So – in a small contradiction to the
Copernican Principle – we do seem to be in a privileged position here on Earth. And humans have made the most of this
position, images taken by Eddington during the solar eclipse of 1919 were
experimental proof against the Newtonian model of gravity and in favour of
Einstein’s theory of general relativity.